The invention relates to disc-shaped or annular seals held by a seal carrier in valves which handle chemically aggressive substances. The seals to which this invention especially relates are multi-layer seals having a sealing surface consisting of PTFE (polytetrafluoroethylene) and an elastic lower layer. The invention further relates to a method of manufacture of seals of the above-described type as well as a method for joining seals of the type described to a seal carrier, i.e., a valve closing member or valve seat.
In valves which are to be used for chemically reactive or aggressive substances and in which the valve drive mechanism and the valve housing are two separate but coupled units, these units are sealed from one another hermetically by a sealing element which can resist the chemically aggressive substances, and a valve closing element which is actuated by the valve actuating mechanism has a seal which is also resistant to the action of the aggressive medium and which opens and closes the valve seat.
In previously known valves employed for handling aggressive media, the seals were mostly homogeneous seals made from solid PTFE or some elastomer which resisted aggressive media and in which the seal was a structural part of the valve closing member.
One disadvantage of seals made from solid PTFE, which does not have an elastomeric quality, is that their sealing function is less effective than that of seals made from elastomers. This disadvantage becomes especially noticeable when the valve seat and the seal are subjected to strenuous service, for example due to very rapid valve opening cycles, or due to a soiling of the medium being handled. As a result, these seals may perhaps be regarded as being fluid-tight but certainly not gas-tight. The above-described disadvantages are especially noticeable in valves which are switched with a low control force, for example magnetically actuated, direct-acting valves.
On the other hand, seals made from an elastic material, i.e., an elastomer, are not generally capable to resist chemically aggressive media so that valves equipped with such elastomer seals cannot be used for handling any and all media.
It has been known in the art to cover objects made from an elastomeric material with a layer of PTFE, for example as described in U.S. Pat. No. 3,511,682 and U.S. Pat. No. 3,514,314. A layer of PTFE of this type was not suitable heretofore in valve seals for aggressive media because of the high permeability of PTFE which permitted the aggressive medium to penetrate it by diffusion and to attack the interior elastomer portion chemically.
It has also been known to improve the resiliency of diaphragms used for electric switch actuation by making a laminate of a layer of PTFE covered glass fiber web and another layer of pure PTFE without fusing the two layers, e.g., as described in British Pat. No. 811,818.